ABYC standard for ELCI's explained

An electrical leak in the shore power system on board a boat results in current returning to its source through the water, potentially exposing swimmers to electrocution and death.  In the latest version of the ABYC standard for boat electrical systems an ELCI is required which will trip the circuit if dangerous levels of current leakage are detected.  An ELCI does for the whole boat what a GFI receptacle does for an outlet in your bathroom;  if you drop the hairdryer in the bath it trips.  The level of leakage required to make the breaker trip is higher in the ELCI because there would be too many nuisance trips otherwise.  For a GFI receptacle the level of leakage required to trip it is 5 mA but for a whole boat ELCI the level is 30 mA.    The ELCI measures the difference between the power coming on board in one wire and going out on the other wire.  If there was no leakage there would be no difference between the two.  If there is leakage some current is returning through the water and so what is going in on one wire doesn't match what is going out on the other. If there is a difference of 30 mA or more it trips.  

Below is the full text of this part of the standard:

ABYC E11.11.1 

An Equipment Leakage Circuit Interrupter (ELCI) or Type A Residual Current Device (RCD) shall be installed with or in addition to the main shore power disconnect circuit breaker(s) or at the additional overcurrent protection as required by E-11.10.2.8.3 whichever is closer to the shore power connection. 

EXCEPTION: Installations where an isolation transformer is installed within 10 feet (3 m) of the shore power inlet or the electrical attachment point of a permanently installed shore power cord and supported according to 11.14.4.1.3 11.11.1.1 The trip level shall be a maximum of 30mA. The trip time shall be a maximum of 100ms. 

11.11.1.2 This device shall meet the requirements of UL 1053 Standard for Safety for Ground-Fault Sensing and Relaying Equipment and the requirements of UL 943 Ground Fault Circuit Interrupters with the exception of trip level, trip time, and grounded neutral detection or 11.11.1.3 IEC 61008, Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses or IEC 61009 Amendment 1 - Residual current operated circuit-breakers with integral overcurrent protection for household and similar uses and IEC 61543, Residual current-operated protective devices (RCDs) for household and similar use - Electromagnetic compatibility. NOTES: 1. Trip levels of less than 30mA and times of less than 100ms may result in nuisance trips in certain environments. 2. If included, the circuit breaker must also meet the requirements of E-11.10.2. 3. IEC compliant RCD’s must be rated for 125, 250 V 60Hz applications. 11.11.1.4 The ELCI or RCD shall be readily accessible.